Photoplethysmography (PPG) is based upon shining light into the human body and measuring how the scattered light intensity changes with each pulse of blood flow. The scattered light intensity will change in time with respect to changes in blood flow or blood opacity associated with heart beats, breaths, blood oxygen level (SpO2), and the like. Such a sensing methodology may require the magnitude of light energy reaching the volume of flesh being interrogated to be steady and consistent so that small changes in the quantity of scattered photons can be attributed to varying blood flow. If the incidental and scattered photon count magnitude changes due to light coupling variation between the source or detector and the skin surface, then the signal of interest can be difficult to ascertain due to large photon count variability caused by loss or variation of optical coupling. Changes in the surface area (and volume) of skin being impacted with photons, or varying skin surface curvature reflecting significant portions of the photons may also significantly impact optical coupling efficiency. Physical activity, such a walking, cycling, running, etc., may cause motion artifacts in the optical scatter signal from the body, and time-varying changes in photon intensity due to motion artifacts may swamp-out time-varying changes in photon intensity due to blood flow changes. Each of these changes in optical coupling can affect the photonic interrogation count by a large percent of the total photon count and diminish the quality of the signal of interest; with lower probability of obtaining accurate values of desired data.
An earphone is a good choice for incorporation of a photoplethysmograph device because it is a form factor that individuals are familiar with, it is a device that is commonly worn for long periods of time, and it frequently is used during exercise which is a time when individuals may benefit most from having accurate heart rate data (or other physiological data). Unfortunately, incorporation of a photoplethysmograph device into an earphone poses several challenges. For example, earphones may be uncomfortable to wear for long periods of time, particularly if they deform the ear surface. Moreover, human ear anatomy may vary significantly from person to person, so finding an earbud form that will fit comfortably in many ears may pose significant challenges. In addition, earbuds made for vigorous physical activity typically incorporate an elastomeric surface and/or elastomeric features to function as springs that dampen earbud acceleration within the ear. Although, these features may facilitate retention of an earbud within an ear during high acceleration and impact modalities, they do not adequately address optical skin coupling requirements needed to achieve quality photoplethysmography.
Conventional photoplethysmography devices, as illustrated for example in FIGS. 1A-1C, typically suffer from reduced skin coupling as a result of subject motion. For example, most conventional photoplethysmography devices use a spring to clip the sensor onto either an earlobe (FIG. 1A) or a fingertip (FIG. 1B). Unfortunately, these conventional devices tend to have a large mass and may not maintain consistent skin contact when subjected to large accelerations, such as when a subject is exercising.
A conventional earbud device that performs photoplethysmography in the ear is the MX-D100 player from Perception Digital of Wanchai, Hong Kong (www.perceptiondigital.com). This earbud device, illustrated in FIG. 1C and indicated as 10, incorporates a spring 12 to improve PPG signal quality. However, the spring 12 forcibly presses the entire earbud 10 within the ear E of a subject to minimize motion of the entire earbud 10. There are several drawbacks to the device 10 of FIG. 1C. For example, the source/sensor module is coupled to the entire earbud mass and, as such, may experience larger translation distances resulting in greater signal variability when the ear undergoes accelerations. In addition, because the earbud 10 is held in place with one primary spring force direction, significant discomfort can be experienced by the end user. Moreover, the earbud motion is only constrained in one direction due to the single spring force direction.